Is it OK to daisy chain battery negative?

[sailorman]

I fitted a Mega fuse in each of the positive cables before the 1/2/both switch at the recommendation of the electrical surveyor, but I wasn't 100% certain what it protects, each downstream load has its own fuse or circuit breaker so I dis wonder what the Mega fuse protects?

protects a large / catastrophic dead short on the battery cable, effectively isolating the batteries

 

[Rivers & creeks]

Makes sense, thanks.